1. Field of the Invention
The present invention relates generally to a projection exposure apparatus and method and, more particularly, to a projection exposure apparatus and method employed when manufacturing a semiconductor device or a liquid crystal display device in a lithography step.
2. Related Background Art
A projection exposure apparatus includes, as a light sending optical system incorporated into a movable wafer stage (a substrate stage), e.g., a movable light sending optical system for illuminating an index mark formed on an index plate on the wafer stage with light beams. The movable light sending optical system illuminates a moving target object such as an alignment mark with the light beams, and is therefore required to have a degree of freedom of light sending. Accordingly the movable light sending optical system involves the use of a flexible light guide composed of a bundle of a multiplicity of optical fibers in order to provide the degree of freedom of light sending.
FIG. 14 is a view schematically illustrating a construction of a prior art projection exposure apparatus including a movable light sending optical system using a flexible light guide.
Referring to FIG. 14, the light beams emitted from a light source 101 common to a main illumination optical system IL for an exposure are, after being reflected by an elliptical mirror 102, incident upon an incident end of a light guide 105 (having one incident end and two emerging ends) via a shutter 103 and a relay lens system 104. A light source image having a sufficient numerical aperture (NA) and diameter is formed at the incident end of the light guide 105.
Fluxes of light beams emerging from the two emerging ends of the light guide 105 respectively form illuminated fields having proper illumination numerical apertures (NAs) through condenser lenses 106, and Kohler-illuminate from under alignment marks 108 formed on stage substrate 107. The illumination light beams passing through the alignment marks 108 illuminate from under a mask 110 (a reticle) through a projection optical system 109. Thus, the illumination light beams passing through the alignment marks 108 are used as alignment light beams for aligning, e.g., the mask and the wafer.
Further, the projection exposure apparatus has, e.g., an illuminance scatter sensor system for measuring an illuminance distribution, in a wafer exposure area, of the main illumination light beams through the projection optical system. FIG. 15 is a view schematically showing a construction of the prior art projection exposure apparatus including the illuminance scatter sensor system.
Referring to FIG. 15, a pin hole 203 substantially flush with an exposure surface of a wafer 201 is formed on the wafer stage 202. Further, the wafer stage 201 incorporates a light receiving sensor 204. The light receiving sensor 204 receives the light beams from an illumination optical system IL via a transparent mask 205 and a projection optical system 206, through the pin hole 203.
The light receiving sensor 204 is connected to a unillustrated processing system. The processing system obtains an illuminance on the basis of a quantity of light beams reaching a light receiving surface of the light receiving sensor 204 through the pin hole 203 for, e.g., one second and an areal size of the pin hole 203. Thus, the illuminance distribution over the exposure area in the wafer 202 can be measured based on an output of the light receiving sensor 204 that is obtained by relatively moving the wafer stage 201, more particularly, the pin hole 203 with respect to the projection optical system 206.
Moreover, in the projection exposure apparatus, an optical system including the light sending optical system and the light receiving optical system that are provided in the movable wafer stage, has a focus calibration mechanism based on, e.g., a contrast detection method. FIG. 16 is a view schematically illustrating a prior art projection exposure apparatus having the focus calibration mechanism based on the contrast detection method.
Referring to FIG. 16, the light beams emitted from a light source 301 common to the main illumination optical system IL for the exposure are, after being reflected by an elliptical mirror 302, incident upon a first bifurcating end 305a of a two-way bifurcation light guide 305 via a shutter 303 and a relay lens system 304. A light source image having a sufficient numerical aperture (NA) and diameter is formed at the first bifurcating end 305a of the light guide 305.
The light beams emerging from an end 305b of the light guide 305 form an illuminated field having a proper illumination numerical aperture (NA) through the condenser lens 306, and Kohler-illuminate from under a transmissive pattern 308 formed on a stage substrate 307 provided on a wafer stage 312. The illumination light beams passing through the pattern 308 are incident on the undersurface of a mask 310 conjugate to a forming surface of the pattern 308 through a projection optical system 309. The light beams reflected by the undersurface of the mask 310 form a pattern image in superposition on the pattern 308 on the stage substrate 307 again through the projection optical system 309.
The light beams from the pattern image passing through the pattern 308 on the stage substrate 307 are incident on the end 305b of the light guide 305 via the condenser lens 306. The light beams emerging from a second bifurcating end 305c after being led to the light guide 305, are received by a light receiving sensor 311 having a light receiving surface located by the second bifurcating end 305c. Then, it is feasible to detect a best focus position with respect to a projection optical system 309.
Thus, there are secured the degree of freedom of the light sending for illuminating the moving target object (the transmissive pattern 308) with the light beams and the degree of freedom of the light receiving for receiving the light beams through the moving target object by use of the flexible light guide 305.
With a higher precision of the projection exposure apparatus in recent years, however, even if the light guide exhibits a high flexibility, an influence exerted on driving accuracies (a stepping accuracy and a scan accuracy) of the wafer stage is going to be unignorable. Further, when the light receiving sensor is incorporated into the wafer stage, and even if a high-performance air-conditioning mechanism is provided around the wafer stage, there can not be ignored influences exerted, by the heat evolved from the light receiving sensor, on the accuracies of other alignment systems as well as on the accuracy of a wafer stage laser interferometer.